clang: Disable unused-function warnings for 3rd-party NCCL library

[kpu]

Actually a lot of warnings from clang are unused functions inside NCCL. Not all of them, but it's a start.

[kpu]

If you're going to express opinions, at least act on them.

"I am not a fan of warnings. I would request that all warnings in our own code are resolved, while it is OK to disable warnings in 3rd-party code after checking them once." --@frankseide
"We should not fix warnings in third-party code. For that, we should instead configure the compiler to not warn." --@frankseide
"As Frank said, either by addressing the issue, or if 3rd-party code or compilers being unreasonable by disabling them. Disabling has to be well motivated though." --@emjotde
"I would be OK to accept this PR (assuming all other comments are addressed), and working on clang on Linux in parallel, if you guys could commit to submit a second PR that fixes any remaining Mac-specific warnings after the clang on Linux has been made warning-free. Would that work for you (Nick, Kenneth, Marcin)?" --@frankseide

[emjotde]

Relax.

[frankseide]

Sorry, did not realize that this was meant for me. Looking now.

[kpu]

Thanks @frankseide

[emjotde]

Reverting for now as this seems to break NCCL compilation with CUDA with g++

[emjotde]

Also confirmed on my local machine, I think the CXXFLAGS overwrite is removing some options?

[kpu]

Feel free to revert; I'll try again on that machine. Funny thing is I tried passing CXXFLAGS through too and that's still broken??

[emjotde]

Could it be it's removing options from the Make? Like Fpic?

[kpu]

Ugh https://github.com/marian-nmt/nccl/blob/b56650c7f59b8cd40d18809784a6d6be38ef8acb/makefiles/common.mk#L46

[kpu]

Actually you just need to update your NCCL submodule: https://github.com/NVIDIA/nccl/blob/master/makefiles/common.mk

[emjotde]

Downgraded on purpose, higher version have a bug for models above 2GB, unfortunately. Mentioned this in Rikhards issue.

We may need to make a special branch in our fork?

[kpu]

Ugh yeah I just re-read #597. Is a special branch of NCCL overkill for compiler warnings on clang?